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The S&P 500 (INDEX: ^GSPC) has seen some big ups and downs so far this year. After rising 12% in the first quarter, the index lost nearly all of those gains before rebounding toward the end of June. For the first half, the S&P posted a rise of more than 8%.

Given the challenges of earning a decent return in the current low-rate environment, an 8% showing for the S&P is nothing to scoff at. Throw in dividends on the S&P, and you'd add another percentage point or so. Yet when you look at the monster performers of the S&P 500 during the first half of 2012, they clearly leave the S&P's single-digit total return in the dust. Let's look more closely at the best stocks in the S&P to see what lessons we can learn from them.

Sears Holdings (NAS: SHLD) -- up 87.9%Looking at Sears shares over the past six months, you might think all of the retailer's woes are behind it. But the real story here is just how depressed the stock was at the beginning of the year. The company had just announced that it would close as many as 120 stores after a terrible holiday season.

TripAdvisor (NAS: TRIP) -- up 77.3%In stark contrast to Sears, TripAdvisor has been an unparalleled success story since its IPO last year. In February, the travel-review site announced 30% revenue growth in its fourth quarter, with huge amounts of traffic from countries around the world. Despite concerns about marketing costs, the company repeated that strong performance in its first-quarter report, beating estimates and predicting revenue growth to hit the top end of its prior guidance range.

PulteGroup (NYS: PHM) -- up 69.6%Homebuilder stocks have been on the rocks for years, as continual disappointment about delays in a true housing recovery has made investors shy away from the space. Yet a warm winter and some signs of economic recovery have finally started pushing shares of homebuilders like Pulte off their lows.

As with Sears, though, it's important to remember that even after these big gains, Pulte's stock has a long way to go before it could even approach price levels from before the financial crisis. Even the hint of bottoming home prices has led to huge enthusiasm, yet investors have gotten burned before from prematurely calling a bottom in the housing market.

Expedia (NAS: EXPE) -- up 66.4%Finally, how often is it that you see both halves of a split-up business do well? After divesting itself of TripAdvisor late last year, Expedia has also seen its core travel-portal business succeed in producing a big share-price gain as well.

In many ways, the move reflects the ongoing trend toward online travel booking versus traditional travel agencies. Rivals in the portal space have also performed well, suggesting that the entire industry has room for growth. As with TripAdvisor, a lot depends on what happens in the crucial summer months, which make or break companies in this business.

Keep on winningFor these stocks, the returns they've earned in six months represent years of gains for many stocks. But for long-term investors, six months is just a blink of an eye, and experienced investors know all too well that what goes up strongly one year can give up all those gains the next. Let me suggest looking more at the good long-term prospects you'll find in the Fool's latest special report, which reveals three Dow stocks with strong dividend payouts and growth prospects. The report is absolutely free, so get your copy today.